1. Field of the Invention
The present invention relates to a polarizer, and particularly to an absorptive wire-grid polarizer.
2. Description of the Related Art
A polarizer in which an one-dimensional metal grating is formed on a substrate with a period smaller than that of a visible light wavelength is called “a wire-grid polarizer”, which has a function as a polarizer by transmitting a polarized light which vibrates in a direction (polarization direction) parallel to a grating period direction and reflecting a polarized light which vibrates in a polarization direction orthogonal to the grating period direction. Moreover, since a wire-grid polarizer is generally formed of inorganic material, it has greater heat resistance and UV resistance comparing to a film type polarizing plate using absorbed pigments. Due to such characteristics, the wire-grid polarizer has advantages such that deterioration of its polarizing function can be suppressed for a long period of time even being irradiated with a light of high light density when used in liquid crystal projectors, and others.
However, since a wire-grid polarizer strongly reflects a polarized light having a polarization direction whose transmission should be suppressed, it is difficult to use the wire-grid polarizer at places where the reflected light becomes a stray light and affects forming of an image (for example, a projected image in a liquid crystal projector). Japanese Patent Laid-Open Nos. 2008-216956 and 2009-300655 disclose examples of absorptive wire-grid polarizer with suppressed reflection. Japanese Patent Laid-Open No. 2008-216956 discloses an absorptive polarizer provided by forming an absorber grating at an upper part of a grating constituting a wire-grid polarizer. On the other hand, Japanese Patent Laid-Open No. 2009-300655 discloses an absorptive polarizer provided by forming an absorber grating and a protective material on a side surface of a convex portion of a dielectric grating.
However, although the absorptive polarizers disclosed in Japanese Patent Laid-Open Nos. 2008-216956 and 2009-300655 have a sufficiently reduced reflectance comparing to those of conventional wire-grid polarizers, they have a problem that a reflectance of 10% or more remains or that an extinction ratio of a polarized light which is transmitted therethrough is low. The extinction ratio is expressed by a ratio of a transmittance of the transmitted polarized light which vibrates in the direction parallel to the grating period direction and a transmittance of the transmitted polarized light which vibrates in the direction orthogonal to the direction parallel to the grating period direction. The extinction ratio is an index indicating a characteristic of the polarizer.
Japanese Patent Laid-Open No. 2009-300655 discloses an optical element ensuring a sufficient extinction ratio by forming a reflective wire-grid polarizer on a substrate surface opposite to another substrate surface on which an absorptive wire-grid polarizer is formed. Even in such a case, although reflection of a light entering the optical element from its absorptive wire-grid polarizer side surface is suppressed, a reflectance of a light entering the optical element from its reflective wire-grid polarizer side surface (back surface) is not reduced. Furthermore, in order to ensure a sufficient transmittance, it is necessary to precisely match transmission axes of the two polarizers formed on the two substrate surfaces, which causes difficulty of manufacturing thereof.